The present invention relates to a plasma processing apparatus in which a sample, that is placed on a sample table arranged within a vessel, is processed using a plasma.
With a recent trend toward higher integration of semiconductor circuits, there has been a demand for the pattern of a circuit to have a finer configuration, and the demand for increased accuracy in the processing dimensions of a semiconductor has been severe. Further, an enhancement of the throughput and the realization of more uniform processing with respect to a larger area of a sample serving as a workpiece, such as a silicon wafer, have been requested as well. Therefore, in a plasma processing apparatus for processing a sample to produce a semiconductor device using plasma, the temperature controllability of a sample serving as a workpiece during the processing is important the following for the following reasons.
For example, in an etching process for formation of a groove having a high aspect ratio, an etching of high anisotropy is employed. In order to realize this, a process has been employed for carrying out etching of a groove bottom, while protecting the sidewalls of the groove by sue of a reactant product, such as an organic material. However, in an apparatus for supplying a processing gas into a vessel, the inside atmosphere of which is at a reduced pressure, the concentration of the reactant product brings forth a distribution under the influence of a characteristic of the exhaust (gas) flow within the vessel during the processing of a sample. Specifically, the reactant product is higher in concentration on the inner peripheral side of a wafer than on the outer peripheral side thereof.
The diameter of a holding surface in an electrostatic chuck on which a wafer is placed within the etching apparatus is sometimes smaller than the wafer itself, for the purpose of protecting the adsorption surface of the electrostatic chuck from the effects of the plasma. In such a case, in the surface near the outer peripheral end of the wafer, the size of a coarse for transmitting heat entering from the plasma, such as an area of the surface or an area of a portion in contact with other members, is small. Therefore, the outer peripheral region of the wafer rises in temperature.
In such a case as described above, particularly, in processing whose processing temperature increases as the wafer temperature increases, a problem arises in that the etching rate of the outer peripheral portion of the wafer becomes higher than that of the inner peripheral portion thereof, so that the etching rate is greatly different within the surface of the wafer. As a technique for improving the distribution of the processing speed, as described, there has been contemplated the provision of means for positively elevating the temperature at a position on the inner peripheral side of the wafer relative to the temperature at a position on the outer peripheral side thereof.
For example, some means have been devised which control the distribution of the temperature within the surface of a workpiece, such as a silicon wafer, that has been chucked on an electrostatic chuck. Among them, Japanese Patent Laid-open No. 251735/1989 discloses a technique in which a gas is filled between an electrostatic chuck and a workpiece, and its pressure (hereinafter referred to as the back pressure or pressure of the back surface) is changed, whereby the heat transfer rate between the electrostatic chuck and the workpiece is changed to control the temperature distribution of the workpiece.
In order to control the temperature, the above-described technique utilizes properties in which the heat transfer rate increases as the pressure of the gas increases. To be more specific, the back pressure of a portion whose temperature should be lowered relatively within the surface of the workpiece, such as a silicon wafer, is made high; whereas, the backpressure of a portion whose temperature should be elevated relatively is made low, thereby causing the heat transfer rate from the workpiece to the electrostatic chuck to have a desired distribution. Thus, a desired temperature distribution in the in-surface direction is created with respect to the amount of heat entering from the plasma above the workpiece.
According to the prior technique, as described above, the temperature distribution of the workpiece, such as a silicon wafer, that is chucked on the electrostatic chuck, can be controlled relatively simply and efficiently in terms of time.
However, there is a problem in that, if a suitable temperature distribution is desirably obtained within the surface of the workpiece, using prior technique as described, it is difficult to control the temperature delicately. More specifically, in the above-described prior technique, where there is heat entering the workpiece, or, where there is a temperature difference between the workpiece and the sample table, including the electrostatic chuck supporting the workpiece, the temperature distribution can be obtained in the process substance. Where the amount of heat entering the workpiece is small, or the temperature difference relative to the sample table (electrostatic chuck) is small, it becomes difficult to form a sufficiently large distribution of temperature. In particular, where the temperature of the sample table or the electrostatic chuck is higher than that of the workpiece, this problem is noticeable. This point will be described in detail hereinafter.
FIG. 6(a) is a flowchart illustrating a flow of processing to be described later, and FIG. 6(b) is a graph showing the pressure distribution of a gap between a wafer and an electrostatic chuck, and a change in the temperature distribution of the surface of a wafer with respect to time. This processing is performed such that, where the back pressure on the inner peripheral side of a workpiece is set to a relatively low value, whereas that on the outer peripheral side is set to relatively high value, in order to control the temperature on the inner peripheral side of the workpiece to be high while the temperature on the outer peripheral side is controlled to be low, in a state where the temperature of the electrostatic chuck in contact with the workpiece to support it is held at a temperature higher than room temperature. Prior to the start of the process, the electrostatic chuck is held at a desired fixed temperature, e.g., 60° C., which is higher than room temperature.
First, a workpiece, such as a semiconductor wafer, whose temperature in the in-surface direction is nearly room temperature and the distribution thereof is set to be generally uniform, is carried to and placed on a sample table provided with an electrostatic chuck. At that time, an electrostatic chucking voltage is applied to the electrostatic chuck substantially simultaneously with the placement of the workpiece. Where the electrostatic chuck is of a single pole type, in this state, plasma having a capacity of static electricity is not formed, and the adhesive force for chucking the workpiece is not created. Therefore, a gas for heat transmission is not introduced into the back of the wafer. A plasma is generated at time t=t0.
Next, heat transmitting helium is introduced between the back of the wafer and the surface of the electrostatic chuck. This helium is introduced to enhance and adjust the efficiency of heat transmission between the wafer and the electrostatic chuck. The pressure of the helium is controlled in such a manner as to be low on the inner peripheral side of the wafer and high on the outer peripheral side, as mentioned above. In such a case, the temperature of the workpiece is normally lower than that of the electrostatic chuck, and the heat transfer rate between the workpiece and the electrostatic chuck is larger on the outer peripheral side than the inner peripheral side; and, therefore, the temperature on the outer peripheral side of the workpiece becomes relatively higher than that on the inner peripheral side with the lapse of time. That is, a temperature distribution that is opposite to the desired temperature distribution is obtained. After the further lapse of time, heat on the outer peripheral side of the wafer is transmitted to the electrostatic chuck more efficiently, and, therefore, the temperature on the outer peripheral side of the wafer gradually becomes lower than the temperature on the inner peripheral side. In this manner, when the time is t=t2, the temperature distributions on the inner and outer peripheral sides are reversed so as to obtained the desired temperature distribution.
As described above, in the prior technique, finally, the desired temperature distribution on the wafer, in which the inside is high and the outside is low, is obtained, but in the actual process, an inconvenience often occurs. In the aforementioned process, because etching is carried out in the period from t0 to t2 with a temperature distribution that is opposite to the desired temperature distribution, and in the process that is adjusted so as to exhibit a predetermined performance of the processing speed or the like where the wafer is provided with the desired temperature distribution, the temperature distributions of the inner and outer peripheral sides of the workpiece are reversed in the midst of the process, and, therefore, for the period of time from time immediately after the processing has been started to t2, a desired processing result is not obtained. Further, where the process is adjusted so as to obtain more suitable processing immediately after starting the process, the result of processing performed thereafter is deteriorated from the initial stage. As described above, adjustment of the process for processing the workpiece to obtain a desired result, for example, construction of a recipe is difficult to impair the performance of processing of a semiconductor for carrying out fine working, which poses a problem in that the yield of the process is lowered. This fact has not been taken into consideration in the prior art.
It is an object of the present invention to provide a plasma processing apparatus that is capable of processing the surface of a workpiece more precisely.